[markwkw]

Isn't the utility of the cycler that it can be a massive object with a lot of shielding for humans and permanently installed life support systems? Like a big space station for whole crews to live safely and comfortably while in transit.

You (incrementally) build up a large cycler in parts, each part accelerated to the orbit once.

Once the cycler is large, it seems infeasible to burn fuel to periodically adjust the orbit since it's massive. Unless propellant less stuff like light sails can be used over long periods.

[jvanderbot]

The benefit of a cycler is the near zero delta V required to maintain it, and the fact that it flies by two interesting orbits, like Mars and Earth.

The consequence of this varies, but includes castles as you suggest.

[adastra22]

Yes, it could be essentially a cruise ship for the journey, and starships are used just to ferry passengers and supplies to/from the cycler at each destination. With life support closure you might not even need much in supplies.